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             <h3>查找重复元素</h3>
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      <pre class="mScroll">
        <code class="lang-javascript">

			function duplicates(arr) {
				const m = new Map()
				for(let val of arr) {
					if(m.has(val)) {
						let cnt = m.has(val)
						m.set(val, cnt+1)
					}
					else {
						m.set(val, 1)
					}
				}
				const res = []
				for(let [key, value] of m) {
					if(value > 1) {
						res.push(key)
					}
				}

				return res.sort()
			}

			console.log(duplicates([2,1,2,1]))
        </code>
    </pre>
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